The invention relates to acoustical insulation particularly designed for use in a motorized vehicle, particularly an automobile.
Nonwoven webs are used in automotive vehicles as an acoustical insulation located in and surrounding the passenger cabin and/or the engine compartment or fire wall. For use in the engine compartment or adjacent fire wall, generally an extremely high temperature stable nonwoven material is required such as mineral wool, fiberglass and the like. However, even around the passenger cabin a certain degree of resistance to thermal degradation is required. The passenger compartments of vehicles can become quite hot, particularly when parked in a sunny location.
It has been found that an extremely efficient acoustical insulation material can be formed by the melt-blown microfiber process, which material is described in U.S. Pat. No. 5,298,694. In terms of cost, processability and performance a preferred blown microfiber web for this use is formed out of polypropylene. However, the problem with polypropylene in the fiber form is that it is susceptible to degradation, particularly thermal degradation.
Polypropylene when it degrades becomes brittle and when in the form of a blown microfiber turns into dust. For example, in U.S. Pat. No. 4,067,836 polypropylene is used to enhance the degradability of a polymer blend by increasing the susceptibility of the blend to be oxidatively degraded. In order to stabilize the polymer composition for a given time period, after which point degradation rapidly occurs, this patent proposes the use of phenol-like antioxidants and others. Similarly, U.S. Pat. No. 5,393,831 describes polypropylene films and fibers which are formed without any phenolic antioxidants. The resulting material is designed to rapidly degrade when exposed to a high temperature environment, see also U.S. Pat. No. 4,038,228.
The exact nature by which any given thermal stabilizer or blend of stabilizers works in any particular instance is often not well understood. However, most if not all thermal stabilizers are considered to be sacrificial. In this sacrificial sense, the stabilizers will preferentially remove essential components involved in the degradation process and are depleted in the process. When all the stabilizers are depleted, the protected materials will then generally rapidly degrade. Further, if the antioxidants or thermal stabilizers are not well distributed throughout the polymer localized degradation can occur. This problem can be compounded where the antioxidants tend to segregate due to incompatibility and/or are of a lower molecular weight such that the stabilizers can migrate and volatilize during use of the article.
Additives, including antioxidants or thermal stabilizers and other conventional additives, are typically added to a polymer prior to feeding the polymer into the extruder when forming either films or fibers. This is also true with respect to melt-blown microfibers as is described in U.S. Pat. No. 5,145,727 and German laid-open publication No. 254378, both of which describe adding additives to polypropylene prior to extrusion and melt-blowing of the fibers. In U.S. Pat. No. 5,145,727 the additives are materials which preferentially migrate to the surface of the melt-blown microfibers. These additives provide specific surface characteristics to the melt-blown microfibers including hydrophilicity, water wettability, alcohol repellency, hydrophobicity, anti-static properties and the like. These types of additives have also been described as sprayed onto the surface of fibers following formation of the melt-blown microfiber web, for example, in U.S. Pat. Nos. 4,753,843; 4,328,279 and RE 31,885.
A very common additive added prior to extrusion of polypropylene in a melt-blowing process is a prodegradent material such as peroxide, see, e.g., WO 96/09428. These peroxides are used to controllably degrade the polypropylene during the extrusion process to provide a suitable molecular weight distribution so as to allow the polypropylene to be efficiently melt-blown into high quality microfiber webs. For example, U.S. Pat. No. 5,271,883 describes peroxides used at levels up to 3000 parts per million in order to control the molecular weight distribution of polypropylene to a range of from 2.2 to 3.5 Mw/Mn and providing a melt flow rate of up to 5000 grams per ten minutes at 230.degree. C. If these peroxides are not added, or sometimes in conjunction with peroxides, polypropylene is generally subject to a controlled thermal degradation in the extruder, as is described in U.S. Pat. Nos. 3,870,567; 4,048,364; 3,849,241 and 5,149,468. This thermal degradation is carried out at quite extreme temperatures as discussed in detail in U.S. Pat. No. 3,849,241. Both forms of degradation help to eliminate the formation of shot or other fiber defects in the melt-blown microfiber web. This controlled degradation also enables the use of higher viscosity and low melt flow rate polypropylene polymers to produce high quality melt-blown nonwoven webs which have average fiber diameters of preferably less than 10 microns. The addition of thermal stabilizers in polymers fed into this control degradation environment would be counter productive. The result would likely be a non-degraded polymer which would produce a lower quality melt-blown nonwoven web with little thermal stability in that all or a substantially all of the antioxidant or thermal stabilizer would be consumed in the extreme degradation environment of the extruder.
German patent No. 25 43 781 proposes thermal degradation where a non-toxic surfactant is used to aid degradation allowing slightly lower thermal degradation temperatures.
The use of antioxidants has been suggested with respect to melt-blowing fibers from polymers which initially have suitable viscosities for melt-blowing. These antioxidants are fed with the polymer into the extruder such as described in Japanese Kokai No. 2-271607, European patent application No. 484952 and U.S. Pat. Nos. 5,288,791; 4,981,747; 5,116,662; 5,169,706; 4,883,549 and 4,707,398.